Over the recent years, as the volume of information required by audio/video equipment has increased, an increased attention has been paid to disk-shaped optical recording mediums, or so-called optical disks, which allow an easy access to data, storage of a large volume of data and superiority in reduction of the sizes of equipment. Optical disks include reproduction-only optical disks, write-once-read-many optical disks, phase-change type optical disks and magnetic optical disks as read-write optical disks, etc., which are called CDs, DVDs, MOs and the like and used in a variety of actual applications. An information layer in such an optical disk, if the disk is a reproduction-only optical disk for instance, is comprised of a synthetic resin substrate, in which convex and concave pit arrays are formed in a spiral arrangement in accordance with recorded information, and a metal thin film of gold, aluminum or other substance which covers the pit arrays on the synthetic resin substrate and serves as a reflection film for reproduction laser light.
Meanwhile, as structure elements of the information layer, a write-once-read-many optical disk or rewritable optical disk comprises a synthetic resin substrate in which a tracking groove pattern is mainly recorded, a recording film such as a dye recording film and a phase-change recording film, and further, in accordance with necessity, a reflection film which reflects reproduction laser light. As the recording film, a thin film which assumes two states that can be detected by means of laser irradiation is used. Information is reproduced, as convex and concave pits formed on a synthetic resin substrate in the case of a reproduction-only optical disk, but marks formed in a recording film on grooves in the case of a write-once-read-many optical disk or rewritable optical disk, reflect reproduction laser light, and as whether there are pits or marks, the length of a pit or mark and the interval between pits or marks are determined from changes in intensity of the reflection light.
These synthetic resin substrates are manufactured by injection molding of a synthetic resin such as polycarbonate, using a metallic molding board of Ni in general which includes a convex and concave pit pattern or groove pattern and is referred to as “a stamper.” An injection molding method permits to quickly manufacture a large number of synthetic resin substrates which contain the same information, using the same stamper. However, since thus manufactured synthetic resin substrates were all transferred from the same stamper, the pit or groove pattern remains entirely identical and so does the information.
This necessitates identification information unique to an individual, for managing optical disks. This is for the purpose of prevention of unauthorized copying is aimed at from a perspective of copyright protection, for example. Such identification information is different between individual mediums, and hence, can not be created using an injection molding method mentioned above. In addition, it must be ensured that the information can not be rewritten with ordinary optical disk recording/reproducing apparatuses which are generally available in the market. Noting this, proposals have been made regarding techniques for recording unique identification information which is different among individuals by a different method other than an injection molding method.
As a method of additionally recording information on a DVD, there is a standard with respect to recording of additional information in a form which resembles a barcode which is an aggregation of diametrical parallel lines. This recording standard stipulates standardized recording areas, modulation scheme, error correction scheme, etc.
This method, for a DVD-ROM for instance, requires to irradiate laser light upon an information layer inside a fabricated optical disk, dissolve and remove a metal thin film which forms the information layer, and define a barcode-like pattern having different reflection coefficients between the removed portions and the surrounding information layer.
There is alternatively a method which does not utilize dissolution of the metal thin film but requires to irradiate laser light upon the substance of the information layer inside the optical disk, change the reflection coefficient by changing a chemical, physical or optical property, and accordingly record information. An optical disk in which a phase-change recording film or a dye recording film is used as the substance of the information layer for instance corresponds to this.
In this manner, it is possible to specify the origin of the optical disk based on the unique information, prohibit illegal copying of the information, and protect the copyright. Further, as the identification information, a production lot number, a production date and the like are written on every disk in some cases, for other purposes such as production management.
A conventional optical disk comprises a first reproduction-only area of BCA (Burst Cutting Area, or sometimes information recorded in BCA) or the like, a second dedicated area formed by pre-pits, and a data area.
The first reproduction-only area is for recording unique information which identifies an individual as described above, while written in the second reproduction-only area is data such as a molding board number for the optical disk and an intensity value of optimal laser light for reading data recorded on this optical disk. The second reproduction-only area is also called “a lead-in area.” If the disk is a reproduction-only optical disk, a content is recorded in the data area, but user information is recorded in the data area if the disk is a read-write optical disk.
However, with the conventional BCA recording method, since BCA recording is executed after a plurality of layers are bonded to each other, recording takes place over a plurality of mutually stacked information layers. This may result in a deteriorated accuracy of reading the unique information which identifies an individual, or even in a complete failure of data read in the worst situation. (In the case of a rewritable optical disk, in some cases, optimization of recording in each information layer becomes difficult, which in turn obstructs perfect crystallization or exerts an influence such as deformation induced by excessive heat.)
In addition, since the area dedicated to BCA recording which is large but has a small capacity gets occupied when the conventional BCA recording method is used, a read-write area which is originally intended for writing of data may shrink and a substantial recording capacity may accordingly decrease. For instance, the occupied area described above is as wide as 1.2 mm in a DVD-ROM.